1. Field of the Invention
The present invention relates to a method for applying a corrosion-protective coating to a joint between corrosion-protectively coated steel pipes which are used in pipelines for town's gas, city water, petroleum, district heating steam, etc.
2. Description of the Prior Art
Generally, steel pipes are used for a piping arrangement for carrying town's gas, city water, petroleum, steam, etc. Because a steel material for such a type steel pipe has a defect of being corroded easily under the presence of water or oxygen, the surface of the steel pipe is generally corrosion-protectively coated with asphalt, coal tar enamel or polyethylene (PE), epoxy, urethane, etc. to thereby provide a corrosion-protective coating layer (referred to as "factory coating layer" because the corrosion-protective coating layer is formed in a factory) on the surface of the steel pipe.
When such corrosion-protectively coated steel pipes having corrosion-protective coating layers respectively are used, there arises a problem that the corrosion-protective coating layers are deteriorated by a high temperature upon welding when piping construction is performed by welding the respective end portions of the corrosion-protectively coated steel pipes. In order to prevent the deterioration of the corrosion-protective coating layers, therefore, the corrosion-protective coating layers in the vicinity of the end portions of the two corrosion-protectively coated steel pipes are removed in advance so that after the end portions are connected to each other by on-site welding, a corrosion-protective heat-shrinkable tube or a corrosion-protective heat-shrinkable sheet is set to cover the outer circumference of the welded portion of the joint and the outer circumference of adjacent portions adjacent to the joint where the surfaces of the steel pipes are exposed because the corrosion-protective coating layers are removed. Then, the heat-shrinkable tube or sheet is heated by propane gas, or the like, so as to be thermally shrunk to thereby corrosion-protectively cover the welded and adjacent portions with the heat-shrinkable material. The corrosion-protective coating method for on-site piping construction is very excellent in corrosion protectivity and a lot of practical examples have been provided because the corrosion-protectively coated steel pipes can be protected from corrosive factors such as water, air, etc. and can be prevented from being corroded by taking the lap between the corrosion-protective coating layer and the heat-shrinkable material sufficiently.
In the aforementioned corrosion-protective coating method, however, the surfaces of the portions which are necessary for corrosion protection, that is, the flat portions (the surface of the corrosion-protective coating layer and the steel pipe surfaces after the removal of the corrosion-protective coating layer) in the two end portions of the corrosion-protectively coated steel pipes, the thickened portion in the welded connection portion, the level-difference portions (between the steel pipe surface and the corrosion-protective coating layer) generated by the removal of the corrosion-protective coating layer, and other portions, are not always smooth and are not even. Accordingly, air often remains in the portion thickened by welding, the level-difference portions, etc. when the corrosion-protective heat-shrinkable tube, or the like, is thermally shrunk. Furthermore, also in the flat portions of the steel pipe surface, the corrosion-protective coating layer, etc. air void is not avoidable at the time of thermally shrinking. Although the amount of such remaining air is too small to cause a problem practically, there are a feeling of uneasiness about progress of corrosion caused by the remaining air, a feeling of uneasiness about safety of strength of the air-remaining portion against mechanical load such as impact, or the like, and so on.
Various methods have been therefore carried out to eliminate air remaining, or the like, in the corrosion-protective coating portion of the heat-shrinkable material. For example, a method in which the level-difference portion is filled in advance with a material such as a sealing material, or the like, mainly containing butyl rubber or a method in which a hot-melt material mainly containing asphalt or butyl rubber is applied in advance onto the flat portions of the steel pipe surface, the corrosion-protective coating layer, etc., is employed. In each of the methods, however, air void cannot be perfectly prevented from remaining. Furthermore, in the latter method, there arise a problem of gas generation and a problem of safety in dealing with high-temperature fluid, or the like, because melting and application of the hot-melt material are necessary in on-site piping construction.